rewriting urls with isapi filters in .net

[Dan]

I have a huge site written in asp and i have now written a isapi filter of
sorts in asp.net to rewrite my dynamic urls to search engine friendly urls
and translate them on iis to the true address.

The problem i have is that it requires the global.asx file to use its
begin_request to tell it to run the rewrite function.

As it is an asp site it wont allow the asx file to be used?

Does anyone have any ideas on how to get round this without turning the
whole site to .net???

Thanks